all that matters is how you file for 2020.
the stimulus is technically an advance of 2020 payments. What that means is the IRS used 2019 (or 2018) to ESTIMATE what you were due in 2020.
But what is on your 2020 tax return (income, dependents, marital status) is what you are ACTUALLY due
if the ACTUAL is greater than the ESTIMATE, the difference is credited to you on Line 30 of form 1040 and will increase your refund or reduce what you owe.
if the ESTIMATE is greater than the ACTUAL, you are not required to give it back and Line 30 will be zero, in fact Line 30 can never be negative.
sounds like you received $1200 and you are due $1800. Line 30 should show $600.
it's that simple.
ps the fact that your spouse does not have an SSN doesn't impact your ability to receive stimulus so suggest not making a decision to file joint or separate on that basis; it won't matter.
